First and second sets of quadrature-amplitude-modulation (QAM) symbols transmitted parallelly in time can differ in the respective patterns of labeling lattice points in the two sets of QAM symbol constellations, which constellation rearrangement approach provides “labeling diversity”. Labeling diversity can lessen the error in reception of transmitted data accompanied by noise, as compared to that in which the same pattern is used to label lattice points of the first and second sets of QAM symbols transmitted parallel in time. First and second sets of amplitude and phase shift keying (ASPK) symbols transmitted parallel in time can use different patterns of labeling to secure labeling diversity.
In Gray mapping of QAM and APSK, the plural-bit labels of immediately adjacent lattice points differ in only a single one of their bits. With regard to Gray mapping, it has been shown that a constellation rearrangement approach improves the performance if two or more versions of the same word are transmitted. The constellation rearrangement scheme for Gray mapping is based on different levels of reliability for the bits, depending on the position of the selected 16-QAM symbols within the constellation. Consequently, the rearrangement rules focus on changing the location of the rearranged version of the 16-QAM symbol to achieve an averaging effect of the levels of reliability. First and second sets of 16 QAM symbols transmitted parallel in time are labeled such that the labeling of each set of 16 QAM symbols bits more likely to experience error in the labeling of each set of 16 QAM symbols is in accordance with the bits less likely to experience error in the labeling of the other set of 16 QAM symbols. For details on constellation rearrangement for 16-QAM Gray mapping, one is referred to U.S. Pat. No. 7,920,645 titled “Data transmissions in a mobile communication system employing diversity and constellation rearrangement of a 16 QAM scheme” granted 5 Apr. 2011 to Alexander Golitschek Edler Von Elbwart, Christian Wengerter and Isamu Yoshi. U.S. Pat. No. 7,957,482 titled “Bit-operated rearrangement diversity for AICO mapping” granted 7 Jun. 2011 to Alexander Golitschek Edler Von Elbwart, Christian Wengerter and Isamu Yoshi describes more extensively the use of labeling diversity for more than one set of 16 QAM symbols transmitted parallelly in time. (AICO is the acronym for “Antipodal Inverted Constellation”.)
In June 2005 a paper “Symbol mapping diversity design for multiple packet transmissions” authored by Harvind Samra, Zhi Ding and Peter M. Hahn was published in IEEE Transactions on Communications, Vol. 53, No. 5, pp. 810 817. The Samra et al. paper presented a simple, but effective method of enhancing and exploiting diversity from multiple packet transmissions in systems that employ nonbinary linear modulations such as phase-shift keying (PSK) and quadrature amplitude modulation (QAM). This diversity improvement results from redesigning the symbol mapping for each packet transmission. Symbol mapping diversity (SMD) requires a small increase in receiver complexity, but provides very substantial reductions of bit error rate when applied to additive white Gaussian noise (AWGN) and flat-fading channels. The general SMD concept was later incorporated in multiple-input/multiple-output (MIMO) and multiple-input/single-output (MISO) communication systems, but was referred to as “labeling diversity”.
Mappings that maximize labeling diversity in 16 QAM symbol constellations were disclosed specifically by Maciej Krasicki in his paper “The essence of 16-QAM labeling diversity” published 11 Apr. 2013 in Electronics Letters, Vol. 49, issue 8, pp. 567-569. Maps for such a mapping will be referred to as optimal-SMD maps in this specification, its accompanying claims, and FIGS. 27 and 28 of its drawings. Krasicki described his work in more detail in a 20 Feb. 2015 open-access on-line Springerlink publication titled “Algorithm for Generating All Optimal 16-QAM BI-STCM-ID Labelings”. This publication focused on 16-QAM labelings of bit-interleaved space-time coded modulation with iterative decoding (BI-STCM-ID), describing an algorithm to generate (without need for random search) optimal labelings for BI-STCM-ID systems with any number of transmit and receive antennas, transmitting over a Rayleigh-fading channel.
Panasonic Corporation sent a paper titled “Enhanced HARQ Method with Signal Constellation Rearrangement” to the TSG-RAN Working Group 1 for discussion during its Meeting #19 held Feb. 27-Mar. 2, 2001 in Las Vegas, Nev., USA. Combining two 16 QAM transmissions with labeling diversity between the labeling of the lattice points in their respective square 16 QAM symbol constellations was reported to provide a 1.2 dB advantage over Chase combining two 16 QAM transmissions without labeling diversity. Combining two 64 QAM transmissions with labeling diversity between the labeling of the lattice points in their respective square 64 QAM symbol constellations was reported to provide a 1.8 dB advantage over Chase combining two 64 QAM transmissions without labeling diversity. Turbo coding rate was ¾ both for 16 QAM transmissions and for 64 QAM transmissions.
COFDM modulation of radio-frequency (RF) signals has been used several years for over-the-air broadcasting of DTV in accordance with the DVB-T and DVB-T2 Standards for Digital Video Broadcasting in several countries other than the United States of America and Canada. COFDM RF signals are being broadcast in the Republic of South Korea and in the United States of America in accordance with an ATSC 3.0 Standard developed by the Advanced Television Systems Committee, an industry-wide consortium of DTV broadcasters, manufacturers of DTV transmitter apparatus, and manufacturers of DTV receiver apparatus. Each of the plural subcarriers of these COFDM signals is amplitude-modulated using 16 QAM or 64 QAM. The COFDM modulation conveys coded data a single time, rather than conveying the same coded data in both lower and upper sidebands of an amplitude-modulation (AM) signal.
In a double-sideband COFDM (or DSB-COFDM) signal, the lower-frequency half and the upper-frequency half of the frequency spectrum convey the same coded data as each other and mirror each other in frequency. The lower-frequency half of the frequency spectrum is commonly referred to as the “lower sideband”, and the upper-frequency half of the frequency spectrum is commonly referred to as the “upper sideband”. Prior-art receivers for DSB-COFDM radio-frequency signals folded the frequency spectrum in half by synchrodyning those RF signals to baseband before applying discrete Fourier transform (DFT) and demapping the resultant quadrature amplitude-modulation (QAM) of COFDM signal subcarriers. The constructive combining of mirrored OFDM subcarriers transpires at QAM symbol level improves the signal-to-noise ratio (SNR) of reception from an additive-white-Gaussian-noise (AWGN) channel by 3 dB. Receivers that demodulate DSB-COFDM RF signals using either single-sideband (SSB) or asymmetric-sideband (ASB) techniques are described in U.S. Pat. No. 10,171,280 titled “Double-sideband COFDM signal receivers that demodulate unfolded frequency spectrum” issued 1 Jan. 2019 to Allen LeRoy Limberg. Limberg prescribed individual discrete Fourier transform (DFT) of the lower and upper halves of the frequency spectrum of the COFDM modulation signal and demapping the resulting sets of QAM symbols from those two halves of that frequency spectrum, then diversity combining their corresponding QAM-lattice-point labels. Maximal-ratio combining soft bits of corresponding QAM-lattice-point labels in accordance with the respective amplitudes of their QAM symbols prior to channel equalization improves SNR of reception over an AWGN channel by 5.5 dB, irrespective of shaping gain. This 2.5 dB better SNR than averaging corresponding QAM symbols is in line with observations concerning multiple-in/multiple-out (MIMO) reception of COFDM modulation signals from plural-antenna arrays, as reported in U.S. Pat. No. 7,236,548 titled “Bit level diversity combining for COFDM system” issued 26 Jun. 2007 to Monisha Ghosh, Joseph P. Meehan and Xuemei Ouyang.
There are other species of COFDM signal, which have asymmetric-sideband (ASB) structural characteristics wherein lower and upper sidebands do not mirror each other in frequency. Such other species can be referred to as asymmetric-sideband COFDM (or ASB-COFDM). The lower and upper halves of the frequency spectrum of ASB-COFDM convey different respective sets of coded data similarly mapped to QAM symbols or to APSK symbols, and these sets of coded data often relate to each other in some way.
COFDM signals that utilizes dual subcarrier modulation (DCM) are described in patent application US-20170104553-A1 published 13 Apr. 2017, titled “LDPC Tone Mapping Schemes for Dual-Sub-Carrier Modulation in WLAN” and claiming an original filing date of 11 Oct. 2016 for inventors Jian-Han Liu, Sheng-Quan Hu, Tian-Yu Wu and Thomas Edward Pare, Jr. DCM modulates the same information on successive pairs of subcarriers. The subcarriers in each successive pair can be similarly separated in frequency to form a split-spectrum COFDM signal with lower and upper sidebands that convey the same coded data. Such separation improves reliability of reception, especially when there are narrow-band interferences. US-20170104553-A1 describes respective mappings of the sets of 16 QAM symbols transmitted parallelly in time, which mappings are similar to each other.
Akram Bin Sediq and Halim Yanikomeroglu described a technique for soft combining soft-bit demodulation results in a paper titled “Performance Analysis of Soft-Bit Maximal Ratio Combining in Cooperative Relay Networks”, which was published in IEEE Transactions on Wireless Communications, Volume 8, Issue 10, (October 2009), pp. 4934-4939. This paper referred to this soft combining technique by the name “soft-bit maximal-ratio combining” or by its abbreviation “SBMRC”. U.S. patent application Ser. No. 15/796,834 filed 29 Oct. 2017 and titled “Communication systems using independent-sideband COFDM” does describe respective symbol constellation arrangements of QAM subcarriers in the lower and upper sidebands of a COFDM signal differing from each other. U.S. patent application Ser. No. 15/796,834 describes this being done in accordance with an SBMRC technique to provide “shaping” gain in addition to diversity gain, owing to bits more likely to experience error in the labeling of each of two sets of QAM symbols corresponding to the bits less likely to experience error in the labeling of the other set of QAM symbols. An SBMRC technique for obtaining signal shaping is described with regard to COFDM signal retransmission in U.S. Pat. No. 9,647,865 titled “Iterative-diversity COFDM broadcasting with improved shaping gain”, granted to Allen LeRoy Limberg 9 May 2017, and claiming a 23 Mar. 2016 priority date. An SBMRC technique for obtaining signal shaping is described with regard to retransmission of PSK or QAM symbol packets in U.S. Pat. No. 7,362,733 titled “Transmitting/receiving apparatus and method for packet retransmission in a mobile communication system”, granted to Noh-Sun Kim et alii 22 Apr. 2008, and claiming a 28 Oct. 2002 priority date.
Besides the SBMRC technique for obtaining shaping gain, there is an ensemble of various techniques to turn constellations with equidistant, equiprobable modulation symbols (like standard square 16 QAM or 64 QAM constellations) into more “Gaussian” constellations. There are two different types of such “constellation” shaping used to provide shaping gain: “geometric” and “probabilistic. “Geometric” amplitude shaping (GAS) employs a uniform distribution (i.e., equiprobable symbols) on non-equidistant constellation points. This entails a change from the standard square 2mQAM constellation. In a GAS constellation, the symbols are by definition not uniformly spaced across the constellation. The in-phase symbols depend on the quadrature symbols and there is no independent processing of in-phase and quadrature symbols. So, GAS usually results in more complex modulation and decoding schemes as compared to “probabilistic” amplitude shaping (PAS), which is based on a pragmatic square QAM modulation scheme. PAS imposes a non-uniform distribution (i.e., non-equiprobable symbols) on a set of equidistant constellation points. PAS relies on the use of a code (called distribution matcher) to gradually vary the probability distribution of the constellation points (from higher probability for the innermost constellation points, to lower probability for the outermost constellation points), resulting in probabilistic shaping of the constellation. PAS can be applied to any constellation type, including square 2mQAM constellations. A type of PAS is described in U.S. patent application Ser. No. 15/374,397 titled “Probabilistic signal shaping and codes therefor” filed 9 Dec. 2016 for Joon Ho Cho et alii, and published 22 Mar. 2018 as US-20180083716-A1. Both geometric and probabilistic shaping techniques offer an SNR gain for the additive white Gaussian noise (AWGN) channel that can closely approach the ultimate 1.53 dB limit posited by Shannon for constellation shaping.
U.S. patent application Ser. No. 15/796,834 also describes respective symbol constellation arrangements of QAM subcarriers in the lower and upper sidebands of a COFDM signal differing from each other, so as to reduce the peak-to-average-power ratio (PAPR) of that COFDM signal. In the past, broadcasters' primary concern with high PAPR of COFDM signal was its costing expensive power bills for linear power amplification in the transmitter. Newer designs of COFDM transmitters for broadcast television improve power amplifier efficiency using variants of the methods described in U.S. Pat. No. 6,625,430 titled “Method and apparatus for attaining higher amplifier efficiencies at lower power levels” granted 23 Sep. 2003 to Peter J. Doherty. Accordingly, PAPR reduction techniques have become less likely to be resorted to. However, the large PAPR of COFDM also causes problems in receiver apparatus that are not avoided and indeed may be exacerbated by using a Doherty method in the broadcast transmitter. These problems concern maintaining linearity in the radio-frequency (RF) amplifier, in the intermediate-frequency (IF) amplifier (if used) and in the analog-to-digital (A-to-D) converter.
A paper titled “Analysis of Coherent and Non-Coherent Symmetric Cancellation Coding for OFDM Over a Multipath Rayleigh Fading Channel” written by Abdullah S. Alaraimi and Takeshi Hashimoto was presented at the IEEE 64th Vehicular Technology Conference held 25-28 Sep. 2006 in Montreal, Quebec, Canada. Symmetric cancellation coding (SCC) was used principally for implementing intercarrier interference (ICI) cancellation, rather than principally for PAPR reduction. However, Alaraimi and Hashimoto's simulations using 2-dimensional modulation of OFDM subcarriers found 0.5 dB lowering of the PAPR of COFDM when symmetric cancellation coding (SCC) was employed. The particular size of the COFDM modulation constellations employed in the simulations was not specified in this paper.
Superposition coded modulation (SCM) is described in detail by Li Peng, Jun Tong, Xiaojun Yuan and Qinghua Guo in their paper “Superposition Coded Modulation and Iterative Linear MMSE Detection”, IEEE Journal on Selected Areas in Communications, Vol. 27, No. 6, August 2009, pp. 995-1004. In SCM the four quadrants of square QAM symbol constellations are each Gray mapped independently from the others and from the pair of bits in the map label specifying that quadrant. Peng et alii studied iterative linear minimum-mean-square-error (LMMSE) detection being used in the reception of SCM and found that SCM offers an attractive solution for highly complicated transmission environments with severe interference. Peng et alii analyzed the impact of signaling schemes on the performance of iterative LMMSE detection to prove that among all possible signaling methods, SCM maximizes the output signal-to-noise-interference ratio (SNIR) in the LMMSE estimates during iterative detection. Their paper describes measurements that were made to demonstrate that SCM outperforms other signaling methods when iterative LMMSE detection is applied to multi-user/multi-antenna/multipath channels.
Jun Tong and Li Peng in a subsequent paper “Performance analysis of superposition coded modulation”, Physical Communication, Vol. 3, September 2010, pp. 147-155, separate superposition coded modulation (SCM) into two general classes: single-code superposition coded modulation (SC-SCM) and multi-code superposition coded modulation (MC-SCM). In SC-SCM the bits in the superposed code layers are generated using a single encoder. SC-SCM can be viewed as conveying a special BICM scheme over successive SCM constellations. In MC-SCM the bits in the superposed code layers are generated using a plurality of encoders supplying respective codewords. MC-SCM can be viewed as conveying special-case multi-level coding (MLC) scheme over successive SCM constellations. (Single carrier modulation is referred to as “SCM” in some texts other than this, but hereafter in this document the acronym “SCM” will be used exclusively to refer to superposition coded modulation.)